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Metallic Expansion Joints >>
Externally Pressurized Expansion Joint
When large amounts of axial
deflection are required in piping systems of relatively high pressure,
bellows which are internally pressurized have limitations due to the
phenomenon called squirm.
This type of expansion joint,
in which the bellows elements are arranged so that they are exposed to
the media on their external surface, and are therefore externally
pressurized, is an ideal unit. Externally pressurizing a bellows
eliminates squirm as a limitation to the design and permits the safe
acceptance of large amounts of axial thermal expansion.
This type of expansion joint has
many other inherent advantages over internally pressurized bellows.
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Features |
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Externally pressurized |
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Permits large amounts
of axial thermal expansion |
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Eliminates squirm |
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CYCLE LIFE AND RATED MOVEMENT
Externally Pressurized Expansion Joint |
PT&P's externally pressurized expansion joints are designed for 1,000 cycles for any one tabulated movement shown. Cycle life may be increased, or movement may be increased (decreasing the cycle life) by utilizing the graph shown on this page. The graph may also be used for superimposing more than one movement condition on the bellows at one time. It is important to remember that the movements shown in the bellows data pages do not allow for any installation misalignment. A proper specification for a bellows expansion joint should reflect what the actual system movements will be. To insure the highest quality bellows for the least cost, all movement conditions must be taken into account. Refer to the HOW TO ORDER section on page 91 for an example on the proper analysis and selection of an expansion joint.
Examples of movement conditions that typically occur in a piping system are:
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INSTALLATION Allow one cycle for installation misalignment.
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START-UP/SHUT DOWN Movements produced between the minimum ambient temperature and the maximum start-up temperature. This is normally a very low number of cycles.
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OPERATING Movements produced due to temperature fluctuations during system operation. This is typically a very small movement for a very large number of cycles.
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EXCURSION Movement resulting from a worst case/ upset scenario, occurring during system operation. This is normally a very low number of cycles
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HOW TO USE GRAPH
- Enter the graph horizontally at the number of anticipated cycles or the movement condition being considered.
- Move vertically until the graph is intercepted.
- Move horizontally and read conversion factor.
Example: Enter graph at 2500 cycles; read C.F. of 1.23 |
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Welder Attaching Lining to Bellows |
Note:Maximum increase in movements for combined movement conditions shall not exceed 130% of catalog rated movements.
Note: Please reference the print version of Piping Technology & Products' Expansion Joint Catalog for reference tables containing technical data on Externally Pressurized Expansion Joints |
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How To Select and Order |
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The first step in selecting the proper expansion joint assembly is to collect and organize as much information regarding the design and operating parameters of the system as possible. As a minimum the following information should be provided:
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Size of the line.
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Design pressure.
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Design temperature.
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Design movements and required cycle life.
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Overall length requirement, if any.
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Type of end connections.
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Bellows material.
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Pipe material.
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Flange specification and material.
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Type of expansion joint.
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Accessories.
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EXAMPLE
1. Assume the following expansion joint design criteria:
a) Line size: 12-inch nominal diameter.
b) Design/operating pressure: 125 psig.
c) Design/operating temperature: 353°F.
d) Movements/cycle life
Installation: 0.25-inch compression; 0.25-inch extension.
Start-up/shut down: 6.0 inches compression; 250 cycles.
Operating: 1.0-inch compression; 2500 cycles.
e) End connections: 150 lb ANSI raised face slip-on flanges: ASTM A-105 (carbon steel).
f) Bellows material: B-168/lnconel 600.
g) Type: Single.
h) Accessories: Drain and purge.
2. Turn to the Conversion Factor Graph, and calculate equivalent catalog movements.
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Total = 5.80
3. Refer to page 93, 12-inch nominal diameter single externally pressurized expansion joints, and go to the lowest pressure rating that is equal to or greater than specified design pressure (125 psig) which will be the 150 psig rated expansion joints. Compare the total equivalent catalog movements to those shown in the catalog, keeping in mind that the tabulated movements shown in the catalog are for 1,000 cycles for one movement only.4. Select the appropriate expansion joint that meets the movement criteria. Since the total equivalent axial movement is 5.80-inches, the unit that is rated for eight inches is the correct selection.5. Include the designations for the type (XS for XFlex/single), drain connection (D), and purge connection (P).
6. The resulting PT&P part number would then be as follows: |
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Get Expansion Joint Pricing Now
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by FAX: (713) 731-8640